1. Field of the Invention
The present invention relates to a control mechanism for controlling a transmission system capable of driving right and left drive wheels at different speeds.
2. Description of the Related Art
In the field of walking operator type lawn mower, one known example, as disclosed in U.S. Pat. No. 4,934,989, has a grass cutting unit mounted between caster type front wheels and engine-driven rear wheels, and control levers extending rearwardly for use by a walking operator in steering the mower. This lawn mower is steerable by operating transmission clutches provided for the rear wheels, respectively. That is, the mower is caused to run straight by driving the rear wheels at the same speed, and to turn round by disengaging one of the transmission clutches to place one of the rear wheels in free rotation state.
However, the mower can turn round only in a fixed way when one of the rear wheels is placed in free rotation state and only the other rear wheel is driven. It is impossible to select a small, sharp turn or a large, gentle turn according to operating conditions.
The technique of turning round a bulldozer or a tank in varied modes is known, in which right and left running devices are driven at different speeds or, when necessary, one is driven forward while the other is driven backward. However, this requires a complex and special control mechanism. It is difficult to apply such a mechanism to a walking operator type lawn mower which is required to repeat a constant speed run and a turnabout frequently for a grass cutting operation.